Обсуждение: How to write a constraint which need to check other table?

In response to ????????? :
> create table a(
>          name varchar(32);
> );
>
> create talbe b(
>           name1 varchar(32);
>           name2 varchar(32);
> );
>
>
> How to write a constraint to check name1, name2 in the table a without change
> table defination?

-- Okay, your tables without constraints:


test=# create table a (name char(32));
CREATE TABLE
test=*# create table b (name1 char(32), name2 char(32));
CREATE TABLE


-- Now add a primary key to table a:

test=*# alter table a add primary key (name);
NOTICE:  ALTER TABLE / ADD PRIMARY KEY will create implicit index
"a_pkey" for table "a"
ALTER TABLE

-- And now adds two foreign keys to table b:


test=*# alter table b add foreign key (name1) references a;
ALTER TABLE
test=*# alter table b add foreign key (name2) references a;
ALTER TABLE

-- show the tables:

test=*# \d a;
          Table "public.a"
 Column |     Type      | Modifiers
--------+---------------+-----------
 name   | character(32) | not null
Indexes:
    "a_pkey" PRIMARY KEY, btree (name)
Referenced by:
    TABLE "b" CONSTRAINT "b_name1_fkey" FOREIGN KEY (name1) REFERENCES a(name)
    TABLE "b" CONSTRAINT "b_name2_fkey" FOREIGN KEY (name2) REFERENCES a(name)

test=*# \d b;
          Table "public.b"
 Column |     Type      | Modifiers
--------+---------------+-----------
 name1  | character(32) |
 name2  | character(32) |
Foreign-key constraints:
    "b_name1_fkey" FOREIGN KEY (name1) REFERENCES a(name)
    "b_name2_fkey" FOREIGN KEY (name2) REFERENCES a(name)


Regards, Andreas
--
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)

In response to ????????? :
> Yes, you are right. That maybe a bad example. what I want to say maybe like
> this:
>
> create table a (
>          id integer,
>          room varchar(32),
>          start time,
>          end time,
>          PRIMARY KEY(id)
> )
> How can I check if it is the same room, when I insert the data, the start time
> and end time doesn't overlap?

Please answer to the list, okay?

test=*# create table a ( id int primary key, room char(32), start_time timestamp, end_time timestamp);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "a_pkey" for table "a"
CREATE TABLE
test=*# create or replace function check_overlapp() returns trigger as $$begin perform id from a where room=new.room
and(start_time,end_time) overlaps (new.start_time,new.end_time); if found then raise notice 'room reserved'; return
null;else  return new; end if; end; $$language plpgsql; 
CREATE FUNCTION
test=*# create trigger trg_check before insert or update on a for each row execute procedure check_overlapp();
CREATE TRIGGER
test=*# insert into a values (1, 'room1', '2009-10-01'::timestamptz, '2009-10-10'::timestamptz);
INSERT 0 1
test=*# insert into a values (2, 'room1', '2009-09-01'::timestamptz, '2009-09-10'::timestamptz);
INSERT 0 1
test=*# insert into a values (3, 'room1', '2009-09-08'::timestamptz, '2009-09-20'::timestamptz);
NOTICE:  room reserved
INSERT 0 0

You can also use RAISE EXCEPTION to force an error:

test=*# create or replace function check_overlapp() returns trigger as $$begin perform id from a where room=new.room
and(start_time,end_time) overlaps (new.start_time,new.end_time); if found then raise exception 'room reserved'; return
null;else  return new; end if; end; $$language plpgsql; 
CREATE FUNCTION
test=*# insert into a values (3, 'room1', '2009-09-08'::timestamptz, '2009-09-20'::timestamptz);
ERROR:  room reserved
test=!#



HTH, Andreas
--
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)